The proposed study will examine the effects of a low fat diet (approximately equal to 20% of calories) on the incidence of breast cancer in a population of women at high risk for breast cancer. Women identified by the Breast Cancer Diagnostic Center (BCDC) of the University of Pittsburgh to have approximately a 1% annual risk for breast cancer and who agree to enter the study, will be randomly assigned to a low fat diet group or a control group with no dietary intervention. The Nutrition Research Group of the Nutrition Division of the Department of Epidemiology in the Graduate School of Public Health at the University of Pittsburgh will perform a pre-randomization interview at the potential participant's home. This interview is designed to allow determination of eligibility based on motivation and ability to substantially change their eating pattern. Once this eligibility has been established, the potential participant will have a physical examination and initial nutritional assessment performed at the Breast Cancer Diagnostic Center. This will include Biochemical measures of nutritional parameters, anthropometric measures and a dietary assessment. A two month period of intense active intervention will be conducted on those patients randomized to the low fat diet group. This will include six home visits supplemented by group session to provide knowledge and the necessary skills to develop and maintain a low fat eating pattern. Follow-up will be conducted every three months at clinic visits and supplemented by group sessions. Compliance will be monitored by FRR scores and measures of lipids or lipoproteins. All patients will have yearly FRR scores and food records. Nutritional adequacy will be monitored and, if necessary, an active intervention to correct significant deficiencies will occur. Physical examination will be conducted on both groups at 6 month clinic visits. Instruction in self-breast examination will be given at each clinical visit. Yearly xeromammograms will be obtained in both control and low fat diet groups.